In a conventional method for surface geometry inspection, known as profilometry, a light plane is projected on the object surface to be inspected. A camera, typically a charge-coupled device (CCD) based camera, is then used to capture the reflection of the light plane on the object. Each profile line is used by an automated system to obtain shape information about the object surface. As the light plane moves relative to the object, a series of profile lines will provide sufficient information to build a 3D model of at least a portion of the object, if desired. The profile line data is used typically for industrial quality control testing, which may involve comparing the 3D model to an expected model.
The CCD acquisition speed limits the maximum permitted speed of movement of the objects to inspect. In fact, an object may move on an assembly line at a speed that will prevent a data acquisition by the CCD, as the acquisition time is significant.
U.S. Pat. No. 6,335,757 describes a method and apparatus for acquiring images at a higher speed than the regular acquisition speed of the CCD, the specification of which is hereby incorporated by reference. This method triggers the CCD to take an image of the substantially horizontally disposed profile line within an essentially dark background at a fast rate such that a multiple j of profile lines can be acquired for each frame consisting of the number k of horizontal lines of the CCD camera. The normal appearance of the frame image data collected from such a system is a series of profile lines separated normally by a number of lines (k/j). The profile lines provide the shape of the object at each acquisition time, and typically the object is moved over time along a path such that the series of profile lines can allow the surface of the object to be reconstructed using the profile lines. By using high-speed profile line acquisition, an object can be accurately scanned while being moved more quickly. In cases where the profiles lines appear continuous and do not change in height at any horizontal point from one acquisition to the next by more than (k/j) lines, the exact profile line can be extracted with all of the height information unambiguously provided.
This prior art method may make it difficult, however, to know the height of the object profile within the field of view. When the profile line changes abruptly from one acquisition to the next, i.e. by an amount more than (k/j) lines, then two profile lines will intersect, and it is very difficult to know what the true position data of the profile line is. For example, an object whose shape changes quickly from having a bump to a depression can result in sequential profile lines overlapping at certain portions. While this problem can be avoided by defining a window height of (k/j) lines and by ensuring that profile lines resulting from scanned objects do not surpass the window height, this will typically require using a smaller value for j, and thus a slower profile line acquisition rate. Alternatively, the projected profile line image can be scaled to be smaller on the CCD surface to avoid overlap with a lower spatial resolution resulting. Reconstruction without assuming that the profile lines fit within predetermined window heights or assuming certain object properties is difficult if not impossible.
There is therefore a need for a method and apparatus that will overcome the above-mentioned drawbacks.